1. Field of the Invention
The invention pertains to microwave impedance measurements and more particularly to the measurement of the complex reflection coefficient at the input port of a microwave network.
2. Description of the Prior Art
In the early days of microwave engineering impedance measurements were performed with the utilization of a slotted section of transmission line. A probe moveable along the slot obtained the positions of the voltage minimums and the ratio of maximum to minimum voltage of the standing wave along the line. From this information the magnitude and phase angle of the reflection coefficient (complex reflection coefficient) of the load impedance was determined and with additional computation the terminating impedance established. Standing wave measurements on a slotted section must be performed one frequency at a time. Consequently, the determination of the impedance or waveguide termination over a broad band of frequencies required repetitive measurements at selective frequencies. This procedure was tedious and in many instances the frequency selection bypassed resonant frequencies of the termination.
An early apparatus for impedance measurements over a continuous frequency band was disclosed in U.S. Pat. No. 2,762,972 issued to Henning and assigned to the assignee of the present invention. In this device the microwave source, as for example a reflex klystron, was mechanically tuned over a specified frequency range by a servo mechanism. The output signal of the klystron was launched on a main waveguide section which was coupled to two appropriately spaced auxiliarly waveguides via cross-guide directional couplers. The first of these auxiliary guides was terminated in a short circuit and a probe on a carriage servo controlled with the tuning mechanism, was inserted in the guide and arranged to track the maximum of the standing wave. The second wave guide was terminated with the test load. A series of probes were inserted in a slotted section of the second waveguide with spacings of one-eighth of a wave length therebetween, and these probes were mechanically coupled for movement with the carriage of the probes in the first auxiliary waveguide in a manner to maintain one probe, the second from the load, a quarter wave length from the test load and to maintain the one-eighth wave length spacing between probes as the frequency changes. In this arrangement the amplitudes of the signals detected by the four probes provide sufficient information for the determination of the complex reflection coefficient of the test load, and thereby its impedance. Though an improvement over the manual test procedure, this system was mechanically cumbersome and did not provide highly accurate impedance measurements over a broad frequency band.
With the advent of high directivity directional couplers and the development of the backward wave oscillator (BWO) rapid reflection coefficient measurements over a wide frequency bandwidth could be made with significantly improved accuracies. In this equipment a BWO was coupled to a waveguide that was terminated with the device under test (DUT). Directional couplers included in the waveguide assembly provided samples of the incident and reflected waves to detector circuitry. Voltage amplitudes of the incident and reflected waves were then coupled to a ratio detector to establish the instantaneous magnitude of the reflection coefficient. Coupling this instantaneous reflection coefficient magnitude and the sweep voltage of the BWO to the x and y terminals of an oscilloscope or recorder provided a visual representation of the reflection coefficient magnitude as a function of frequency. Though such a system provides valuable information concerning the waveguide termination, due to the lack of the reflection coefficient phase information, it does not provide sufficient data from which the impedance of the load may be determined.
Complex reflection coefficients may be obtained with BWOs by replacing the detector circuitry with appropriately arranged quadrature phase detectors. With this arrangement inphase and quadrature components of the reflected wave relative to the incident wave may be determined. The resulting inphase and quadrature components may then be combined to establish the magnitude and phase of the waveguide termination. In this system the bandwidth of the measurement is limited by the bandwidth of the directional couplers and the quadrature phase detectors.
Another method employed in the prior art utilizes a six port network, having four output ports and two input ports. The detected powers at the four output ports may be combined to determine the input power at the first and second input ports and the real and imaginary parts of the complex power determined from the complex input signal at one input port multiplied by the complex input signal at the second input port. The impedance of the waveguide load is then determined by taking the square root of the sum of the magnitudes of the two input powers, to establish the magnitude of the reflection coefficient, and by taking the ratio of the imaginary complex power to the real complex power, to establish the tangent of the phase angle of the reflection coefficient. This method of computing the reflection coefficient of a waveguide termination may be performed over a broad continuous band, but requires a significant degree of computation and concomitantly significant computation time.
Though methods exist in the prior art for the determination of the impedance or reflection coefficient of a waveguide load, these methods either require extensive computation or provide only the magnitude of the reflection coefficient. There is a clear need for an apparatus that provides the magnitude and phase of the reflection coefficient with the same simplicity as the measurement of the magnitude alone.